1. Field of the Invention
The present invention relates to a piezoelectric resonator and an FM detection circuit incorporating the same.
2. Description of the Related Art
A phase shifter of an FM detection circuit which detects changes in the frequency of an FM wave by detecting changes in voltages has conventionally been used in a discriminator. Generally, to achieve a wide demodulation output bandwidth, piezoelectric materials that produce a low Q and a wide bandwidth xcex94F (=Faxe2x88x92Fr) are used for the discriminator. However, the relationship of various temperature coefficients of the piezoelectric materials has not been appropriate, resulting in a relatively large temperature coefficient (foTC) of the finished product. Thus, the guaranteed operation temperature range of a device incorporating the discriminator has been narrower than discriminators that incorporate a ceramic filter, which prevents the use of the discriminator in various devices.
The temperature coefficient (foTC) of a discriminator in a finished product has been on the order of 25 ppm/xc2x0 C., which corresponds to a frequency change of approximately 28 kHz in a temperature range of 100xc2x0 C. and approximately 40 kHz in a temperature range of 150xc2x0 C. Furthermore, in discriminators which have previously been available, frequency change has tended to be greater at temperatures above 20xc2x0 C. Thus, the upper limit of the guaranteed operation temperature range has often been set to 60xc2x0 C. in order to meet general specifications for foTC which assume a change of xc2x1300 kHz in fo.
In order to counter the problems described above, Japanese Unexamined Patent Application Publication No. 63-283215 discloses a device in which a capacitor is connected in series to a discriminator (piezoelectric resonator) and the temperature coefficient of the capacitance of the discriminator and the temperature coefficient of the capacitance of the capacitor satisfy a predetermined relationship, such that a change in the frequency-impedance characteristics associated with a temperature change in the discriminator is cancelled by the temperature characteristics of the capacitor, thus reducing a shift in frequency.
Furthermore, Japanese Registered Utility Model No. 2501521 discloses a bridge circuit with resistors connected respectively on three sides thereof and a discriminator (piezoelectric resonator) connected on the remaining side, in which a capacitor having temperature characteristics equivalent to those of the discriminator connected in parallel to one of the resistors.
However, each of the proposals requires use a capacitor in addition to a discriminator and thus requires control of the temperature characteristics of the capacitor which increases the uncertainties. Thus, it has been difficult to provide an FM detection circuit which achieves desired temperature characteristics.
To overcome the above-described problems, preferred embodiments of the present invention provide a piezoelectric resonator and an FM detection circuit incorporating the same, in which various temperature coefficients of the piezoelectric material are optimized, such that a finished product has stable temperature characteristics and a guaranteed operation temperature range that is greatly increased.
According to a preferred embodiment of the present invention, piezoelectric resonator is provided, wherein the temperature coefficient xcex5TC of the capacitance of the piezoelectric material, the bandwidth ratio xcex94f/fo, the temperature coefficient FrTC of the resonance frequency, the temperature coefficient FaTC of the anti-resonance frequency, and a target value xcex1 for the temperature coefficient of the center frequency satisfy the following expression:
|(FrTC+FaTC)/2+Kxc3x97xcex5TCxc3x97(xcex94f/fo)|xe2x89xa6xcex1xe2x80x83xe2x80x83(1) 
where K=a coefficient determined according to the impedance at the midpoint between Fr and Fa; xcex5TC=Axc3x97(the amount of change in capacitance in a measured temperature range)/(the capacitance at a reference temperaturexc3x97the measured temperature range); xcex94f/fo=(Fa at the reference temperaturexe2x88x92Fr at the reference temperature)/(fo at the reference temperature); FrTC=Axc3x97(the amount of change in Fr in the measured temperature range)/(Fr at the reference temperaturexc3x97the measured temperature range); FaTC=Axc3x97(the amount of change in Fa in the measured temperature range)/(Fa at the reference temperaturexc3x97the measured temperature range); and A=a coefficient of +1 for a positive temperature coefficient and xe2x88x921 for a negative temperature coefficient.
In accordance this preferred embodiment, because the piezoelectric material is selected such that the temperature coefficient of the capacitance and the temperature coefficient of the anti-resonance frequency cancel each other, the amount of change in the center frequency fo associated with a temperature change is greatly reduced, i.e., the temperature coefficient foTC of the center frequency fo is reduced. Thus, the piezoelectric resonator has a wider guaranteed operation temperature range which produces a wider guaranteed operation temperature range in a device incorporating the piezoelectric resonator. Furthermore, because a capacitor for improving temperature characteristics need not be connected separately, the structure is greatly simplified and desired temperature characteristics are achieved.
In a piezoelectric resonator that is sealed by a packaging resin, in addition to the temperature coefficients of the piezoelectric resonator, the temperature coefficient RfoTC of the center frequency related to a stress of the packaging resin is also taken into consideration such that the following expression is satisfied:
|(FrTC+FaTC)/2+Kxc3x97xcex5TCxc3x97(xcex94f/fo)+RfoTC|xe2x89xa6xcex1xe2x80x83xe2x80x83(2) 
Accordingly, the effects of the temperature coefficient of the packaging resin are eliminated, such that stable temperature characteristics are achieved in a piezoelectric resonator sealed by a packaging resin.
Another preferred embodiment of the present invention provides a method of calculating a temperature coefficient of a piezoelectric resonator, wherein the temperature coefficient foTC of the center frequency is calculated according to the following approximate expression from the temperature coefficient xcex5TC of the capacitance of the piezoelectric material, the bandwidth ratio xcex94f/fo, the temperature coefficient FrTC of the resonance frequency, and the temperature coefficient FaTC of the anti-resonance frequency:
foTc=(FrTC+FaTC)/2+Kxc3x97xcex5TCxc3x97(xcex94f/fo)xe2x80x83xe2x80x83(3) 
where K=a coefficient determined according to the impedance at the midpoint between Fr and Fa; xcex5TC=Axc3x97(the amount of change in capacitance in a measured temperature range)/(the capacitance at a reference temperaturexc3x97the measured temperature range); xcex94f/fo=(Fa at the reference temperaturexe2x88x92Fr at the reference temperature)/(fo at the reference temperature); FrTC=Axc3x97(the amount of change in Fr in the measured temperature range)/(Fr at the reference temperaturexc3x97the measured temperature range); FaTC=Axc3x97(the amount of change in Fa in the measured temperature range)/(Fa at the reference temperaturexc3x97the measured temperature range); and A=a coefficient of +1 for a positive temperature coefficient and xe2x88x921 for a negative temperature coefficient.
In a piezoelectric resonator sealed by a packaging resin, in addition to the temperature coefficients of the piezoelectric resonator, the temperature coefficient RfoTC of the center frequency of a stress of the packaging resin is also taken into consideration so that the temperature coefficient foTC of the center frequency is calculated according to the following expression:
foTC=(FrTC+FaTC)/2+Kxc3x97xcex5TCxc3x97(xcex94f/fo)+RfoTCxe2x80x83xe2x80x83(4) 
Accordingly, the temperature coefficient foTC of a piezoelectric resonator can readily be calculated from the temperature coefficient xcex5TC of the capacitance of the piezoelectric material, the bandwidth ratio xcex94f/fo, the temperature coefficient FrTC of the resonance frequency, and the temperature coefficient FaTC of the anti-resonance frequency, which facilitates circuit design.
The target value xcex1 for the temperature coefficient of the center frequency is preferably 18 ppm/xc2x0 C. More specifically, assuming the center frequency fo=10.7 MHz, if the temperature characteristics foTC of the center frequency of a piezoelectric resonator in a finished product is within xc2x118 ppm/xc2x0 C., which corresponds to a frequency change of approximately xc2x129 kHz in a temperature range of 150xc2x0 C., an operation temperature range of, for example, xe2x88x9240xc2x0 C. to 105xc2x0 C. is guaranteed. That is, compared with the prior art in which the upper limit of the guaranteed operation temperature range is 60xc2x0 C., the upper limit is increased to 105xc2x0 C. according to the present invention.
The coefficient K determined according to the impedance at the midpoint between Fr and Fa is, for example, 0.225. In a piezoelectric resonator in which the center frequency fo is set where the impedance is 1 kxcexa9, using K=0.225, the difference between the temperature coefficient foTC of the center frequency and the average of the temperature coefficient FrTC of the resonance frequency and the temperature coefficient FaTC of the anti-resonance frequency is substantially proportional to the product of the temperature characteristics xcex5TC of the capacitance and the bandwidth ratio, which allows for an accurate calculation of the temperature coefficient foTC of the center frequency.
Another preferred embodiment of the present invention provides an FM detection circuit including a bridge circuit with resistors connected on three sides thereof and a piezoelectric resonator as described above connected on the remaining side, wherein an FM intermediate-frequency signal is input across one of the pairs of opposite nodes of the bridge circuit and output across the other pair of opposite nodes.
Accordingly, the temperature characteristics of the center frequency fo are stable which serves to increase the guaranteed operation temperature range of an FM detection circuit.
Other features, characteristics, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings.